Butted contacts have been widely used for connecting semiconductor devices. Occupying less layout area, butted contacts are particularly suitable for laying out integrated circuits requiring high density such as static random access memory (SRAM) circuits.
An example of the usage of butted contacts is illustrated in FIGS. 1 and 2. FIG. 1 illustrates a schematic circuit diagram of a conventional 6T SRAM cell. The 6T SRAM cell comprises a pass gate transistor 10 and a latch, which includes transistors 12, 14, 16 and 18. Gate 1 of the pass gate transistor 10 is controlled by a wordline WL that determines whether the current SRAM cell is selected or not. The latch, formed of pull up transistor 12, pull down transistor 14, and transistors 16, 18, stores a state. The stored state can be read through a bitline BL.
Butted contacts can be used on various connections in the circuit shown in FIG. 1. For example, the connection between the source region 2 of the pass gate transistor 10 and gate 6 of the transistors 16 and 18. Other connections that can be formed of butted contacts include connections between gate 6 of the transistors 16 and 18 and respective drains 4 and 8 of the transistors 12 and 14, and connections between gate 21 of the transistors 12, 14 and drains 15, 23 of the respective transistors 16 and 18.
FIG. 2 illustrates a cross-sectional view of a butted contact 42 formed between the source region 2 of the pass gate transistor 10 and a gate 6, or an extension of the gate 6, of the transistors 16 and 18 (not shown). Region 40 is a shallow-trench-isolation (STI). The conventional butted contact 42 suffers drawbacks. Due to etching loading effect during the formation processes, the butted contact 42 often has a non-vertical profile with the bottom portion narrower than the top portion, as illustrated in FIG. 3, and the bottom portion may have little or no overlapping portion 44 with the source region 2 of the transistor 10. As a result, the contact resistance increases. Even worse, if overlapping portion 44 completely disappears, an open circuit occurs, causing integrated circuit failure.
Therefore, there is the need for a novel butted contact design that overcomes the above-discussed problems.